1. Field of the Invention
The present invention relates to a light emitting diode and, more particularly, to the improvement in the optical output distribution in a light emitting diode array which is used for photoprinters and the like.
2. Description of the Related Art
As a light emitting diode (hereinunder referred to as "LED") array with an improved optical output distribution, one having the structure shown in FIG. 7 is well known. The LED array is composed of a substrate 52, light emitting regions 54, an electrode 53, individual electrodes 56, etc. The substrate 52 has a two-layer structure having, for example, an n-type GaAsP 52a layer on the upper side and, for example, an n-type GaAs layer 52b on the lower side. On the under surface of the substrate 52 is provided the electrode 53 made of a gold (Au) alloy or the like.
On the upper surface of the substrate 52 (layer 52a), a multiplicity of light emitting regions 54 made of p-type diffusion layers are arranged at regular intervals. An insulation layer 55 is formed over the upper surface of the substrate 52 except at the light emitting regions 54 so as to insulate the upper surface of the substrate 52. On the insulation layer 55, a multiplicity of separate electrodes 56 made of Al or the like are provided. As shown in FIG. 8, one strip-like conductor portion 56c is connected to the end portion of each of the separate electrodes 56 in such a manner that the conductor portion 56c extends over the light emitting region 54 until the end portion 56b thereof reaches the insulation layer 55.
The LED array shown in FIG. 7 is obtained by improving an LED array in which ohmic contact is established only between the end portion 56b of the separate electrode 56 and the end portion of the light emitting region 54. Since the conductor portion 56c transverses the light emitting region 54 in the direction of the Y-axis, the current density in the light emitting region 54 as a diffusion layer is made uniform, but it is still insufficient for realizing high-quality printing by a photoprinter. In other words, in an LED array each LED provided with one strip-like conductor portion 56c (especially, an LED array having a low dot density), the distribution of the the emission intensity in the direction Xc (in the transverse direction of the light emitting region 54 of the each LED) shown in FIG. 5(c) becomes non-uniform, as indicated by the fine dot line in FIG. 5 (d).
This is because it is difficult to efficiently apply a current from the conductor portion 56c which is situated above the light emitting region 54 to both ends of the light emitting region 54. As a result, the gradient of the emission intensity indicated by the dotted line Xc in FIG. 5(d) becomes gentle at the portions closer to both ends of the light emitting region 54. Therefore, the width of a dot d printed by a photoprinter with the emission intensity characteristic of Xc is narrower in the direction of Y.
To solve this problem, the ratio b/a of the width a of the light emitting region 54 and the width b of the conductor portion 56c in FIG. 8 may be increased, but if the dimension of b is too large, a part of the printed dot is deficient (blank). It is therefore difficult to optimize the ratio b/a to make the emission intensity uniform.